fix(studio): make the volume fader tell the truth about the gain it writes

[miguel-heygen]

The volume fader travels in dB, so its stops are irrational values. Serializing them through the generic two-decimal numeric formatter collapsed the bottom quarter of its travel onto "0" — a hard mute — and made the knob jump on release everywhere below unity. Both property panels now use an exact serializer that round-trips every integer fader stop back to itself.

The automation lane reaches the same ceiling

Clamping the volume lane at unity meant automating a boosted clip silently discarded the boost, and the panel disables the fader while a lane owns the level — so there was no way back short of deleting the lane.

This rescales the lane's vertical axis: unity now sits about a quarter of the way up rather than at the top, because the axis covers the same range the fader does. Worth knowing before you open a lane.

New lint rule

audio_volume_tween_overrides_gain. Tween values on volume are absolute — they replace the authored gain rather than scaling it — so a clip carrying both plays at whatever the tween names, and the fader gives no sign of it. The rule reuses the tween detector the sibling lane/tween rule already has, and reports the authored gain in dB.

Verifying

The fader's precision is pinned by a test that walks all 201 stops and asserts none of the audible ones serialize to silence and each round-trips to the same stop. bun run lint on a composition with data-volume="1.949845" and a volume tween emits exactly one warning naming the element and 5.8 dB.

Stack

Based on u1-audio-gain-core, which introduces the shared gain module. Retarget to main before merging.